Electrical property investigations and microstructure characterization of a nuclear borosilicate glass ceramic

Abstract

The electrical properties of a borosilicate glass ceramic used as a conditioning matrix for nuclear waste management are investigated by impedance spectroscopy over frequency and temperature ranges from 1 Hz to 1 MHz and from room temperature to 500 °C respectively. The microstructure of the glass ceramic was studied by scanning electron microscopy which revealed a phase separation phenomenon. The single circle arcs obtained from a complex impedance plot ($-Z^{\text{'}\text{'}}\left(\omega \right)vs{Z}^{\text{'}}\left(\omega \right)$) indicate a pure ionic conduction mechanism and no Maxwell-Wagner-Sillars (MWS) polarization effect. The dependence of ac conductivity on frequency follows Jonscher's power law (known as the UDR: Universal Dielectric Response) very well. The plateau at low frequency corresponding to dc conductivity (${\sigma }_{dc}$ or ${\sigma }_0$) is mainly associated with the long-distance motion of alkali ions and increases linearly with temperature in Arrhenius coordinates. Scaling of both conductivity and dielectric properties were performed to avoid electrode polarization contribution and to provide better understanding of the relaxation behaviour in this sample. The master curves obtained from Summerfield scaling and normalized peaks ($Z^{\text{'}\text{'}}\left(\omega \right)/Z_{max}^{\text{'}\text{'}}$ and $M^{\text{'}\text{'}}\left(\omega \right)/M_{max}^{\text{'}\text{'}}$) formalism suggest a conductivity due to short and long range mobility for the lowest temperatures and only a long-range mechanism associated with a non-Debye relaxation process at higher temperatures.

Introduction

For many years, studies of sodium borosilicate glasses have attracted great interest because of their use in many technological and scientific applications. Their chemical durability and low coefficient of thermal expansion justify their use in laboratory glassware and glass sealing, respectively [1,2]. Another important application is found in the nuclear field for the management of high-level waste (HLW) [3,4].

During the last decades, research and investigations have been undertaken to improve HLW management by increasing the capacity of vitrification and by the conditioning of a wider range of waste types. Nowadays, the cold crucible induction melter (CCIM) technology, whose principle is based on heating glass by direct electromagnetic induction at high frequency close to 300 kHz and cooling the inner walls to create a protective solid state glass layer, has partially replaced the previous melters in certain facilities all over the world and some other countries are about to implement this technology in their vitrification plants [[5], [6], [7], [8], [9]]. Thanks to this new process, melter life has been extended and higher temperatures can be reached. This opens the way to the consideration of new compositions for nuclear waste matrices. The electrical properties of the glass are of primary interest and need to be optimized in order to master the CCIM vitrification process: while the glass melt has to be conductive, the solid glass at the walls, called the “self-crucible”, has to be insulating especially at the working frequency around 300 kHz.

Impedance spectroscopy (IS) is a powerful method for the characterization of charge transport and relaxation processes occurring in many kinds of solid or liquid state materials [10,11]. Previous investigations based on IS applied to different nuclear glasses have revealed that electrical conduction is mainly related to ion mobility, as has been observed in various ionic materials [10,[12], [13], [14]]. Other studies have highlighted the impact of some of the platinum group metals (PGM), such as ruthenium dioxide (RuO₂) incorporated with the fission products, on glass electrical conductivity. These undissolved elements involve an electronic contribution to electrical conductivity depending on the PGM/glass volume ratio [[15], [16], [17], [18]]. As far as is known, no studies focussing on the dielectric properties of nuclear glasses have been performed, although localized relaxation phenomena in the self-crucible must be understood in order to estimate their contribution to local heating and to characterize the insulating properties of the solid glass.

In this paper, a full study of the electrical properties of a simulated nuclear glass ceramic (GC) is reported. IS measurements were used to investigate the charge transport mechanisms at different scales in a sodium borosilicate glass ceramic developed for CCIM applications, from 100 °C to the glass transition temperature. To complete the study, the microstructure of the sample material was also characterized by XRD and SEM-EDS techniques.